A variety of vehicles including farm tractors, track-driven excavators and the like, employ matching hydraulic systems which individually incorporate an operatively associated pump and a motor for driving the opposite sides thereof for maneuvering purposes. While these hydraulic drive systems are generally very effective, most of these vehicles have a tendency to veer or drift from a straight line course because of the problems of synchronizing the drive motors.
In the past, in order to assure straight line travel of the vehicle, sophisticated and expensive mechanisms have been developed to mechanically connect the pumps together so that equal flow will be directed to the vehicle driving motors. However, these mechanisms require precise and frequent adjustment because of external wear thereof and also because of internal wear of the pumps and motors. Furthermore, valve leakage variations are simply not taken into account by such mechanisms. These mechanisms are designed to be ineffective when turning the vehicle or otherwise steering it away from a straight line course which adds significantly to their complexity.
Because these hydraulically operated vehicles employ a number of hydraulically operated implements, it is desirable to power such auxiliary equipment by utilizing the same fluid systems associated with the motors for propelling the machine. Consequently, for maximum effectiveness and economy of operation, open loop circuits are frequently utilized and any synchronizing technique that is used to assure straight line travel of the vehicle must be compatible with the operation of these open loop circuits and their associated auxiliary equipment.